Mesures usuelles. An anniversary remembered

The comment by Han Maenen on the previous article reminds us that this is the 200th anniversary of a decree which ended temporarily the use of the metric system for everyday purposes in France and elsewhere.

In brief, the introduction of the metric system into France in the late 1790’s was poorly managed. This, combined with lack of understanding, made it unpopular. Many people still thought in non-decimal terms using the fractional subdivisions of the old system. Napoleon saw the difficulty of gaining general acceptance for a decimal system of measures and introduced a ‘half-way-house’ system of measurement, mesures usuelles, for use in retail. His decree implementing this change was dated 12 February 1812. Government continued to use the metric system and it was taught at all levels of education.

Pre-revolutionary had France ‘enjoyed’ a wide diversity of measures. Contemporaries estimated that under the cover of some eight hundred names, ancient regime France contained a staggering 250 000 different units of weights and measures. These had been swept away by the revolution, just as the UK Weights and Measures Act of 1824 swept away many of Britain’s medieval measures. Napoleon was left with no option but to define mesures usuelles in terms of the prototypes of the metre and the kilogram, a precedent followed much later in the US in 1893 and in the UK in 1963. As an example of the new system, the toise was defined as two metres, just as the Imperial yard is now defined as 0.9144 metres, and was divided into 6 new pieds and 72 new pouces.

Some ask if the UK should follow even further in the steps of Napoleon. After all, the UK has now adopted several of the features of mesures usuelles, including the definition of imperial measures for length, mass and volume in terms of metric standards, and an attempt to run two measurement systems in parallel – the “very British mess” with which we are familiar.

The metric pound would directly correspond to the French livre and would be very similar in size to some of the historical pounds that were once used in parts of Europe. The metric pint (500 mL) would be almost the same size as the US liquid pint (473 mL), albeit slightly bigger. These units would just be redefined to be rational metric sizes. The old definitions would be abolished.

The advantage of this approach is that consumers who want to continue to use the words ‘pounds’ and ‘pints’ could do so.”

UKMA takes the view that compromises such as these would complicate and prolong the metric changeover, and should be avoided. The country needs one system of measurement, not two . Successive UK Governments have made many mistakes during the metric transition, but thankfully ‘mesures usuelles’ have not been among of them.

History too has not treated mesures usuelles kindly. No other country took up the idea, the decimal metric system was re-introduced permanently in the Low Countries in 1820, and the system of mesures usuelles was dropped in France on 1 January 1840.

It was, arguably, the Swiss that paid the highest price for Napoleon’s bright idea. During the French Revolutionary War, Switzerland had been refashioned by the French as the Helvetian Republic. By 1812, its transition to the metric system was almost complete. Mesures usuelles saw to it that Switzerland reverted to its old measures, and its changeover to metric was not resumed until the 1870s.

But the most interesting question relating to mesures usuelles is perhaps that put by Han Maenen in his recent comment on Metric Views: Would the US have avoided its current measurement muddle, Mars Orbiter, Route I-19 and all that, if Napoleon and France had persevered with the introduction of the decimal metric system beyond 1812?

5 Responses to Mesures usuelles. An anniversary remembered

The National Bureau of Standards (now NIST) published an interesting history of weights and measures in the US as Special Publication 447. It is out of print but available as free pdf download from NIST. Those interested may wish to read it and form their own opinion. My viewpoint after reading SP447 follows:

I think it hinges on whether the Federal government would have adopted metric in or around 1832. The colonies continued to use pre-Revolutionary measure after the Revolutionary War (using various "standards" of debatable accuracy). The early Federal government talked a lot but did a poor job of exercising its Constitutional authority to fix weighs and measures. Pressure to do so increased in the 1820's.

In 1832, the Federal goverment adopted the British yard, and (troy) pound, the 231 in³ gallon (Queen Anne wine gallon) and 2150.42 in³ (Winchester) bushel. These were a continuation of what the States had been doing since before the War but gave a common and central definition. Both the metric system and the various 1824 Imperial improvements were rejected as too different from the prevailing practice. The US had length and weight standards from Britain for the yard and troy pound (av. pound was established by ratio) but also had meter and kilogram standards since about 1820. (The US Coastal Survey dept. used that meter bar as the standard for all their work). "States rights" vs the Federal government were a huge issue in this timeframe. I'm not sure the Federal government could have done anything beyond harmonizing slightly conflicting standards.

Given that we rejected Imperial in 1832, I think we would have rejected metric, even apart from Napoleon, but there is no way to be sure.

The Metric Act of 1866 was primarily a response to foreign trade issues, but formal adoption of metric was considered a number of times after that but always (narrowly) failed. The Mendenhall order in 1893 was a response to quality issues with our yard and pound standards vs the new meter and kilogram standards we had received as a result of the 1875 Treaty of the Meter.

I would note that mesures usuelles were convenient metric approximations to traditional measure. The definitions adopted in the Mendenhall Order (and I believe in the UK in 1963) were based on best available measurement and very minor rounding; they were inconvenient numbers of four or more significant figures.

And this is why we shouldn't let laymen decide on issues best left to the scientific community. It's because bad ideas such as this we use tonne instead of the correct megagram, bar instead of pascal, horsepower instead of watt an so on.
The purpose of the metric system is to ease communication between different cultures and peoples. When you reapply old measurements with new metricated variants, you add more confusion. There should be no such thing as the metric gallon or metric pint. Actually there should be no such thing as the liter, since we have m^3, but then I might be asking too much.

Mårten (above) makes an interesting point. I don't entirely agree with him as it happens. I *do* agree that there should be no *official* endorsement of bogus units like "metric pound", though I don't see why "I'm going for a pint" can't be said even in reference to beer being sold in 500ml measures. Actually, in the UK right now that's what's already happening if your local pub is selling you a beer it doesn't have on draught....

But though "tonne" is indeed maybe an unnecessary alternative to Mg, the idea of a named unit to represent the m² and another to represent the m³ seems to be a good idea to me. With SI as it stands, you can't apply the standard prefixes to areas and volumes properly because you can't say a "millisquaremetre" or a "kilocubicmetre".

Trouble is, the units we *do* have for areas (the are) and volumes (the litre) have appeared as accidents of history and don't represent the 1m² base area or 1m³ base volume. We can and do apply the standard prefixes to them, but it would have been more rational to have had the unit called the "are" mean 1m² (not 10m x 10m as present, and maybe the "vol" representing 1m³.

*Then* (but only then) we would not need the litre (it would be the "millivol" instead).

But by far the most fundamental fix that SI needs to apply to itself is to find a unit name for mass. Maybe to go back to calling it a "grav" as was originally intended? The basic unit of mass should not have a standard prefix already applied....

There is an obsolete unit, the stere, for the cubic meter. It is not part of the SI as it was withdrawn some time between 1948 (9th CGPM) and 1960 (11th CGPM), where its symbol (st) is no longer defined.

I don't see much problem with area and volume as squared and cubed lengths. To have usable ranges, it is necessary to use the "unloved" prefixes, centi, deci, deka, and hecto, but they attach to the length unit and are squared or cubed with it. In engineering calculations, it is usually necessary to treat areas and volumes as squared and cubed lengths, in particular the liter and milliliter normally need to be recognized as cubic decimeters and cubic centimeters, respectively. Otherwise, coherence is lost.

In particular, "kiloarea" would be a problem as the square root of 1000 is not a power of ten. The hectare is OK only because of the strange definition of the "are." The hectare is also 1 hm². However, a kilare would be 3.162 277 66 hm², quite inconvenient. As the CGPM accepts liter, tonne, and hectare, and they are easily expressable in SI units, I think we should just accept them. I do think prefixes greater than 1 should not be used with the liter, and prefixes less than one with the tonne and are. (I think the current SI Brochure only accepts the hectare, and NOT the are, with prefixes other than hecto.)

I do agree the kilogram causes some confusion and renaming it might be appropriate. I would hate to call it the grav, however, due to possible association with gravity. While kilogram-force is completely deprecated in the SI, people insist on using it and it recreates the confusion between mass and weight. Lets not go there.